1. Field of the Invention
The present invention relates to a centrifugal air blower, and more particularly to a centrifugal air blower for use in an air-conditioning system in a vehicle.
2. Description of the Related Art
One conventional centrifugal air blower is illustrated in FIGS. 5 and 6 of the accompanying drawings. The conventional centrifugal air blower 1 in FIGS. 5 and 6, has a fan 3 rotatable by a motor 2 and a scroll casing 5 having a spirally shaped first discharge passage 4 extending around the fan 3 for forcing air into the passenger's compartment of a vehicle, for example, upon rotation of the fan 3. The scroll casing 5 comprises a main case 5a housing the motor 2 and the fan 3 therein, and an extended case 5b attached to the main case 5a. 
The motor 2 is fixedly mounted on the main case 5a so as to extend through a lower wall of the main case 5a. The fan 3 is mounted on the rotatable shaft of the motor 2 and is accommodated substantially centrally in the main case 5a. The fan 3 comprises a plurality of blades 6 spaced at equal angular intervals around the circumference thereof, an annular holder ring 7a mounted on the upper ends of the blades 6, and a bottom plate 7b mounted on the lower ends of the blades 6.
The main case 5a comprises an upper plate 9 disposed above the fan 3 and having an air inlet 8 defined therein over the fan 3, a lower plate 10 disposed beneath the fan 3 vertically facing the upper panel 9, and an arcuate outer wall 11 joining the circumferential edges of the upper and lower plates 9, 10. As shown in FIG. 5, of the main case 5a, the cross-sectional area near a terminal position N of the first discharge passage 4 progressively increases toward the outlet, and the distance from the center of the fan 3 to the outer circumferential edge of the first discharge passage 4 progressively increases toward the outlet of the first discharge passage 4. The lower plate 10 comprises an annular panel 12 positioned near the bottom plate 7b, a spiral flat panel 13 disposed substantially perpendicularly to the outer wall 11, and a spiral joint panel 14 slanted downwardly from the annular panel 12 toward the outward direction of the fan 3.
The extended case 5b has a second discharge passage 15 defined therein which communicates with the outlet of the first discharge passage 4. The cross-sectional area of the second discharge passage 15 progressively increases toward its outlet from the outlet of the first discharge passage 4.
For reducing the overall size of the conventional centrifugal air blower 1, the second discharge passage 15 in the extended case 5b, for example, is vertically increased therefore allowing the scroll casing 5 to be compact. In this case, the vertical size of the second discharge passage 15 is increased from a position of the outer and inner walls, which can be connected by a line, i.e., from a position X of the outer wall and a position Y of the inner wall. Thus, the four sides of the vertical cross section of the second discharge passage 15 are formed with lines.
As shown in FIGS. 5 and 6 of the accompanying drawings, the extended case 5b defining the second discharge passage 15 therein, has an extended upper panel 16 which progressively spreads upwardly at an angle θUP from a position near a terminal position N of the spirally shaped main case 5a, and an extended lower panel 17 which progressively spreads downwardly at an angle θLR from an end portion of the shaft of the motor 2. The highest end of the extended upper panel 16 is higher that the upper panel 9 by a distance HUP, and the lowest end of the extended lower panel 17 is lower than the annular panel 12 of the lower plate 10 by a distance HLR.
If the ratio HUP/HLR of the distance HUP, which is the distance from the outer end of the extended upper panel 16 and the upper panel 9, and the distance HLR, which is the distance from the outer end of the extended lower panel 17 to the annular panel 12 of the lower plate 10, is large, then swirling flows 18, 19 (see FIG. 6) are generated near the position X where vertical size of the outer wall starts to increase and near the position Y where vertical size of the inner wall starts to increase, causing an energy loss of the air that flows through the first and second discharge passages 4, 15. As a result, a large pressure loss may occur in the conventional centrifugal air blower 1, and the conventional centrifugal air blower 1 tends to produce large noise.
If the positions X, Y are displaced upstream of the first discharge passage 4, then the angle θLR, by which the extended lower panel 17 is progressively spread downwardly, is reduced, possibly reducing the swirling flows 18, 19 produced at the positions X, Y. However, since the vertical positional difference between the bottom plate 7b of the fan 3 and the flat panel 13 is increased, a swirling flow 20 is liable to be developed. Consequently, the amount of air discharged from the conventional centrifugal air blower 1 is reduced due to a pressure loss caused thereby, and increased noise is generated by the conventional centrifugal air blower 1.